God first gave Abraham his word, Genesis 18:10, then he confirmed it by his oath, Genesis 22:16. The apostle, to the Hebrews, saith, Hebrews 6:13, When God made promise to Abraham, because he could swear by no greater, he swore by himself.
Related Commentaries of Luke 1:73
Luke 1:73
73 The oath which he sware to our father Abraham,